Various lock mechanisms have heretofore been provided for releasably securing one member in a selected position of rotary adjustment relative to a second member; however, because of certain inherent structural characteristics, these mechanisms are beset with one or more of the following shortcomings: (a) the mechanism is of costly and complex construction; (b) the mechanism is readily susceptible to malfunction and requires an inordinate amount of servicing and repair; (c) the mechanism is difficult to operate and is unreliable; and (d) the mechanism requires an awkward and time-consuming manipulation to effect locking and/or unlocking of the mechanism.